The effervescence you observe when hydrochloric acid (HCl) reacts with zinc (Zn) is caused by the formation of hydrogen gas (H₂). Here's a breakdown of the reaction:

1. The Reaction:

* HCl + Zn → ZnCl₂ + H₂

This is a single displacement reaction where zinc replaces hydrogen in hydrochloric acid.

2. The Formation of Hydrogen Gas:

* Zinc (Zn) is more reactive than hydrogen (H). When zinc comes in contact with HCl, the zinc atoms donate electrons to the hydrogen ions (H+) in the acid.

* This results in the formation of zinc chloride (ZnCl₂) and hydrogen gas (H₂).

* The hydrogen gas is released as bubbles, causing the effervescence.

3. Effervescence:

* Effervescence is the rapid formation of bubbles in a liquid.

* The bubbles of hydrogen gas are less dense than the liquid solution, so they rise to the surface and escape into the air.

In summary: The effervescence during the reaction of HCl and Zn is due to the production of hydrogen gas, which forms as a result of the single displacement reaction between the two reactants.